Method and system for extended volume imaging using MRI with parallel reception

ABSTRACT

A method and apparatus for producing an image from an extended volume of interest within a subject using a Magnetic Resonance Imaging (MRI) system are provided. The method comprises translating the volume using a positioning device along an axis of the MRI system. A plurality of MR signals are detected from at least one radiofrequency (RF) coil array for a given field-of-view within the MRI system as the positioning device is translated. The plurality of MR signals are sent to a plurality of receivers wherein the receivers are each adapted to adjust their respective center frequencies at a rate commensurate with a rate of translation of the positioning device. A plurality of respective sub-images corresponding to the plurality MR signals for each of the plurality of receivers are combined to form a composite image of the volume of interest.

BACKGROUND OF INVENTION

[0001] This invention relates to a method and system for medicalimaging. More particularly, this invention relates to a method andsystem for extended volume imaging using a Magnetic Resonance Imaging(MRI) system and employing a moving patient table.

[0002] Certain clinical situations require that a head-to-toe scan of apatient be made. For example, metastatic cancer can occur anywhere inthe body and patients at risk for metastatic disease need to beevaluated regularly. Currently head-to-toe or, alternatively, extendedvolume imaging is generally performed with whole body positron emissiontomography (PET) systems or nuclear studies in which small amounts ofradioactive substances are given to the patient and allowed to collectin regions of rapid tumor growth. The imaging sensitivity andspecificity of magnetic resonance (MR), however, makes MR a moredesirable choice for diagnosis particularly when Gadolinium (Gd)contrast agents are employed. Gd contrast agents are typicallyadministered intravenously and tend to collect in regions ofangiogenesis associated with active tumors. Unfortunately, MR scannershave limited sensitive volumes and whole body scanning requires that aseries of images be made at different stations. Patient motion and tableregistration can make the joining of these separate images a challengeand tend to create image artifacts referred to as “stitching artifacts”.

[0003] Generally, imaging using a MRI system involves imaging a volumeof interest in a MRI scanner's usable volume. The usable volume isdefined as a contiguous area inside the patient bore of a MagneticResonance scanner and it can be limited in size. Typically, when theusable volume fails to cover an extended object, a method for examiningthe whole volume containing the object employs repeated executions ofpositioning and imaging a fraction of the whole volume within thescanner's usable volume to obtain regional images. A subsequentassembling operation then assembles or “stitches” the regional imagestogether to produce a final image of the whole volume of interest. Suchan approach is typically challenged by the “stitching” artifact issuewherein resulting final images often suffer from distinctive artifactsat the boundaries of the “stitched” pieces.

[0004] Existing techniques achieve correct combination of regionalimages through full spatial encoding along patient table motiondirection. With other existing methods, the patient table is heldstationary while data is collected and moved between the collection ofthe regional images. These techniques minimize “stitching” artifacts byusing slab selection profiles that are as rectangular as possible,and/or discarding image data near the boundaries. As a result, thesetechniques tend to be inflexible, require prolonged radio frequency (RF)excitation, and involve considerable acquisition efficiency degradation.

[0005] More recently, imaging an extended volume is performed by meansof simultaneous patient table translation thereby allowing examinationof a field of view that extends beyond the usable volume of an MRscanner. It is however very difficult to achieve 3-dimensional wholebody coverage with favorable mapping accuracy, spatial resolution,signal-to-noise ratio and total scan time.

[0006] Parallel imaging offers a way to speed up conventional MRimaging. The idea of detecting MR signal with multiple coils receivingin parallel has been explored. Recent advances represented bysimultaneous acquisition of spatial harmonics (referred to as SMASH) andsensitivity encoding (SENSE) make up for a reduced number ofgradient-driven spatial encodes by integrating data from an array ofreceive RF coils. SMASH and the likes assume a frequency perspectivethey fill up skipped k-space lines through approximating Fourierharmonics with linearly combined coil sensitivity profiles. SENSE andrelated methods adopt a space perspective they resolve localizationambiguities through algebraically extracting additional spatialinformation encoded with the sensitivity profiles.

[0007] What is needed is a method and system for extended volumeimaging, such as head-to-toe imaging, using a MRI system to reduce theimaging time while offering good image quality.

SUMMARY OF INVENTION

[0008] In a first aspect, an imaging apparatus for producing MagneticResonance (MR) images of a subject is provided. The apparatus comprisesa magnet assembly for producing a static magnetic field, a gradient coilassembly for generating a magnetic field gradient for use in producingMR images, at least one radiofrequency (rf) coil array disposed aboutthe subject for transmitting a radiofrequency pulse and for detecting aplurality of magnetic resonance (MR) signals induced from the subject, apositioning device for supporting the subject and for translating thesubject into the magnet assembly, and, a plurality of receivers forreceiving the plurality of MR signals, the receivers each being adaptedto adjust their respective center frequencies at a rate commensuratewith a rate of translation of the positioning device.

[0009] In a second aspect, a method for producing an image from anextended volume of interest within a subject using a Magnetic ResonanceImaging (MRI) system where the extended volume of interest is largerthan an imaging portion of a magnet within the MRI system is provided.The method comprises translating the volume using a positioning devicealong an axis of the MRI system. A plurality of MR signals are detectedfrom at least one radiofrequency (RF) coil array for a givenfield-of-view within the MRI system as the positioning device istranslated. The plurality of MR signals are sent to a plurality ofreceivers wherein the receivers are each adapted to adjust theirrespective center frequencies at a rate commensurate with a rate oftranslation of the positioning device. A plurality of respectivesub-images corresponding to the plurality MR signals for each of theplurality of receivers are combined to form a composite image of thevolume of interest.

BRIEF DESCRIPTION OF DRAWINGS

[0010] The features and advantages of the present invention will becomeapparent from the following detailed description of the invention whenread with the accompanying drawings in which:

[0011]FIG. 1 illustrates a simplified block diagram of a MagneticResonance Imaging system to which embodiments of the present inventionare useful;

[0012]FIG. 2 is a side-view block diagram of a receiver arrangement foruse in the MRI system of FIG. 1 and to which embodiments of the presentinvention are applicable;

[0013]FIG. 3 is a simplified graphical illustration of a dataacquisition sequence to which embodiments of the present invention areapplicable; and,

[0014]FIG. 4 is a simplified graphical illustration of a trans-axialview of a RF coil array for use in the MRI system of FIG. 1 and to whichembodiments of the present invention are applicable.

DETAILED DESCRIPTION

[0015]FIG. 1 illustrates a simplified block diagram of a system forproducing images in accordance with embodiments of the presentinvention. In an embodiment, the system is a MR imaging system whichincorporates the present invention. The MR system could be, for example,a GE-Signa MR scanner available from GE Medical Systems, Inc., which isadapted to perform the method of the present invention, although othersystems could be used as well.

[0016] The operation of the MR system is controlled from an operatorconsole 100 that includes a keyboard and control panel 102 and a display104. The console 100 communicates through a link 116 with a separatecomputer system 107 that enables an operator to control the productionand display of images on the screen 104. The computer system 107includes a number of modules that communicate with each other through abackplane. These include an image processor module 106, a CPU module108, and a memory module 113, known in the art as a frame buffer forstoring image data arrays. The computer system 107 is linked to a diskstorage 111 and a tape drive 112 for storage of image data and programs,and it communicates with a separate system control 122 through a highspeed serial link 115.

[0017] The system control 122 includes a set of modules connectedtogether by a backplane. These include a CPU module 119 and a pulsegenerator module 121 that connects to the operator console 100 through aserial link 125. It is through this link 125 that the system control 122receives commands from the operator that indicate the scan sequence thatis to be performed. The pulse generator module 121 operates the systemcomponents to carry out the desired scan sequence. It produces data thatindicate the timing, strength, and shape of the radio frequency (RF)pulses which are to be produced, and the timing of and length of thedata acquisition window. The pulse generator module 121 connects to aset of gradient amplifiers 127, to indicate the timing and shape of thegradient pulses to be produced during the scan. The pulse generatormodule 121 also receives subject data from a physiological acquisitioncontroller 129 that receives signals from a number of different sensorsconnected to a subject 200, such as ECG signals from electrodes orrespiratory signals from a bellows. And finally, the pulse generatormodule 121 connects to a scan room interface circuit 133 which receivessignals from various sensors associated with the condition of thesubject 200 and the magnet system. It is also through the scan roominterface circuit 133 that a positioning device 134 receives commands tomove the subject 200 to the desired position for the scan.

[0018] The gradient wave forms produced by the pulse generator module121 are applied to a gradient amplifier system 127 comprised of G_(x),G_(y) and G_(z) amplifiers. Each gradient amplifier excites acorresponding gradient coil in an assembly generally designated 139 toproduce the magnetic field gradients used for position encoding acquiredsignals. The gradient coil assembly 139 forms part of a magnet assembly141 which includes a polarizing magnet 140 and a RF coil 152. Volume 142is shown as the area within magnet assembly 141 for receiving subject200 and includes a patient bore. As used herein, the usable volume of aMRI scanner is defined generally as the volume within volume 142 that isa contiguous area inside the patient bore where homogeneity of main,gradient and RF fields are within known, acceptable ranges for imaging.A transceiver module 150 in the system control 122 produces pulses thatare amplified by an RF amplifier 151 and coupled to the RF coil 152 by atransmit/receive switch 154. In an embodiment of the present invention,transceiver module 150 comprises a plurality of receivers that will bediscussed in greater detail with reference to FIG. 2. The resultingsignals radiated by the excited nuclei in the subject 200 may be sensedby the same RF coil 152 and coupled through the transmit/receive switch154 to a preamplifier 153. The amplified MR signals are demodulated,filtered, and digitized in the receiver section of the transceiver 150.The transmit/receive switch 154 is controlled by a signal from the pulsegenerator module 121 to electrically connect the RF amplifier 151 to thecoil 152 during the transmit mode and to connect the preamplifier 153during the receive mode. The transmit/receive switch 154 also enables aseparate RF coil (for example, a head coil or surface coil) to be usedin either a transmit or receive mode. It is to be appreciated that RFcoil 152 is configured to be operable for MRI scanning as describedbelow, in which a subject is translated on a positioning device alongthe z-axis. As used herein, “adapted to”, “configured” and the likerefer to mechanical or structural connections between elements to allowthe elements to cooperate to provide a described effect; these termsalso refer to operation capabilities of electrical elements such asanalog or digital computers or application specific devices (such as anapplication specific integrated circuit (ASIC)) that is programmed toperform a sequel to provide an output in response to given inputsignals.

[0019] As is well-known, RF coil 152 is used for detecting MR signalsfrom the region of interest that is to be imaged, e.g. whole-body coil,surface coil or head coil. To image an extended volume, a whole-bodycoil is desirably used for RF coil 152. A whole-body coil may be aconventional and well-known birdcage coil configuration or,alternatively a variety of coil structures comprises a plurality ofconductors formed in a substantially circular form interconnectedstructurally and electrically that is disposed about a subject to beimaged. The conductors are desirably of sufficient length to detectsignals in a desired field-of-view as is also well-known. Forembodiments of the present invention, RF coil 152 is a receive coilarray comprising a plurality of elements arranged in an array and willbe described in greater detail with reference to FIG. 4.

[0020] The MR signals picked up by RF coil 152 are digitized by thetransceiver module 150 and transferred to a memory module 160 in thesystem control 122. When the scan is completed and an entire array ofdata has been acquired in the memory module 160, an array processor 161operates to Fourier transform the data into an array of image data.These image data are conveyed through the serial link 115 to thecomputer system 107 where they are stored in the disk memory 111. Inresponse to commands received from the operator console 100, these imagedata may be archived on the tape drive 112, or they may be furtherprocessed by the image processor 106 and conveyed to the operatorconsole 100 and presented on the display 104. As will be discussed withreference to embodiments below, further processing is performed by theimage processor 106 that includes reconstructing acquired MR image dataaccording to embodiments described below.

[0021] Referring to FIG. 2, there is shown a side-view block diagram ofa receiver arrangement for use in the MRI system of FIG. 1. As shown inFIG. 2, subject 200 is positioned on positioning device 134 of the MRscanner. As used herein, MR scanner refers generally to the MRI system.Subject 200 is translated through the useable volume (142 of FIG. 1) ofthe scanner at a constant rate until the entire body of the subject (ordesired portion thereof) has passed through magnet assembly 141 (FIG.1).

[0022] Thus, an imaging apparatus for producing Magnetic Resonance (MR)images of a subject is provided. The apparatus comprises a magnetassembly for producing a static magnetic field; a gradient coil assemblyfor generating a magnetic field gradient for use in producing MR images;at least one radiofrequency (rf) coil assembly for transmitting aradiofrequency pulse and for detecting a plurality of MR signals inducedfrom the subject; a positioning device for supporting the subject andfor translating the subject into the magnet assembly; and, a pluralityof receivers for receiving the plurality of MR signals and being furtheradapted to be adjusted in phase or frequency changes in response totranslation of the positioning device.

[0023] Multiple MR sub-images of portions of the subject are made whilethe subject is moved through the imaging portion of the MR imagingmagnet. MR signals are detected from RF coil 152 and simultaneously sentto a plurality, N, of receivers which are shown as 250, 251 and 252 inFIG. 2. The description which follows corresponds to an exemplaryembodiment in which N=2, and the receivers are receiver 250 and receiver251. It is to be appreciated that further embodiments would comprisemultiple receivers and image processing computations described belowwould be adapted by one skilled in the art for the selected number N ofreceivers. At the completion of subject's 200 movement through magnetassembly 141 (FIG. 1) the sub-images are processed by image processor106 (FIG. 1) and combined to form a composite image of the entiresubject.

[0024] The receivers are desirably configured to be adjusted in phase orfrequency in response to phase or frequency changes due to translationof the positioning device. During scanning either the phase or thefrequency of the receivers (or transmitter) is continuously changed tomatch the phase and frequency changes of spin magnetization caused bypatient motion through the geometrically-fixed gradient system of themagnet.

[0025] If it is desirable for the frequency-encoding direction to beparallel to the axis of subject motion, then it is the receiverfrequency that is changed at a rate commensurate with the rate oftranslation of the positioning device (hereinafter “table motion”). Therate of frequency change can be determined from the table motion rate(in cm/sec), the gyromagnetic ratio of the nuclear spins (approx. 4250Hz/Gauss for 1H) and the scan rate (TR) (in ms). The table speed can beassumed to be slow during the short periods during data acquisitionitself (typically 4-8 ms). In further embodiments, however, the receiverfrequency is varied during data acquisition. If the frequency directionis chosen to be parallel to the table motion, then the optimalfield-of-view (FOV) for the frequency-encoding direction will be thesize of the RF coil divided by N.

[0026] If the operator desires that the phase-encoding direction of theimage acquisition be parallel to the direction of table motion, then thereceiver phase is changed as a function of an incremented position ofthe positioning device (hereinafter “table position”). In this case theoptimal FOV of the phase-encoding direction of each sub-image would betwice the size of the RF coil to prevent phase-wrap artifacts. If it isdesirable for the slice-selection direction of the image to be parallelto the table motion (as in an axial slice), then the transmitterfrequency is changed to cause the excitation slice to move through themagnet at the same rate that the subject moves. In this case there areno practical restrictions on the FOV of the sub-image.

[0027] It should also be noted that oblique scanning is possible byperforming simple matrix rotations to the gradient subsystem and to thereceiver and transmitters frequencies in a manner well known to thoseskilled in the art.

[0028] Additionally, an additional phase or frequency offset isdesirably added to each receiver causing the reconstructed data to beshifted by a distance corresponding to the field-of-view divided by N.In a exemplary embodiment of the invention in which thefrequency-encoding axis of each sub-image is parallel to the tablemotion, the acquisition of a single sub-image is complete in 1/Nth thetime it takes the subject to traverse the sensitive imaging volume ofthe magnet. Furthermore, the order of data acquisition can be modifiedso that k-space is traversed twice during the acquisition of eachsub-image (e.g. odd numbered rows in the first half of the sub-imagescan, even numbered rows in the second half). This aspect will bedescribed in greater detail below and with reference to FIG. 3.

[0029] Once a sub-image is collected the phase or frequency of thereceiver is reset, and a subsequent sub-image at an adjacent locationwithin the subject is acquired. The process is repeated until thesubject's entire body has passed through the magnet and has been imaged.

[0030] Because each receiver operates at a different phase or frequency,images acquired with each receiver will contain signals from differentportions of the body. The fields-of-view of each receiver overlap eachother, however because the relative offsets are FOV/N. Consequently,every part of the subject's body is acquired in a central portion of onesub-image.

[0031] In the exemplary embodiment of the invention, N=2 and 256 linesof k-space are acquired with the frequency direction of the acquisitionapplied in the direction of table motion. The frequency of bothreceivers is changed at a rate that causes image acquisition to beperformed at the same anatomical location during the acquisition of asub-image. Furthermore, the sub-images acquired with the two receiversare offset from one another by ½ the field-of-view.

[0032] With this embodiment the sequence of data acquisition is asfollows: Receiver 1 Receiver 2 Sub image # k-space Sub image # k-space 1odd — — 1 even 1.5 even 2 odd 1.5 odd 2 even 2.5 even 3 odd 2.5 odd 3even 3.5 even

[0033] In this illustrative example which is shown in FIG. 3, thesub-image number represents both the time sequence and the relativelocation of the sub-image in the context of the subject's anatomy.

[0034] Note that the same RF pulses and magnetic field gradient pulsesare used to simultaneously generate data from the subject for bothreceivers. Because there are N=2 passes through k-space, the pulses usedto acquire the second half of sub-image 1 are also used to acquire thefirst half of sub-image 1.5.

[0035] Once all the data has been acquired, an MR image of the entirebody is created by combining the central portions of each sub-image.Since each receiver operates at a unique offset, every portion of thesubject is imaged in a central portion of a sub-image and discontinuityartifacts associated with image edge boundaries (“stitching artifacts”)are minimized.

[0036] Alternatively, the final composite image is created by combiningfull sub-images to obtain a composite image with an enhancedsignal-to-noise ratio. In this embodiment, each point in the subject'sanatomy is found in N sub images. Since these N images have onlypartially correlated noise, combining the full images will provide asignal-to-noise enhancement. For N=2, the noise in each sub-image is ½correlated since ½ of the data in each sub-image is identical. Thus, theexpected SNR gain would be sqrt(3/2) since the data acquired isequivalent to a 1.5 NEX acquisition. Conversely, if N=3, then theexpected SNR gain would be sqrt(5/3) or equivalent to a NEX=5/3acquisition. In general the SNR gain for using N receivers is given bythe expression:

[0037] Sqrt((2N−1)/N).

[0038] As N becomes large, the SNR gain approaches sqrt(2) or that of aNEX=2 acquisition (i.e. the SNR gain of imaging twice as long).

[0039] A further alternative embodiment employs a single receiver thatis operated with the frequency direction applied parallel to the tablemotion. In this case, the field-of-view of the receiver is set to beequal to a selected active imaging volume less than or equal to theimaging volume. At the beginning of the scan the center of the sub-imageis set to be ¼^(th) of the way into the active volume of the imagingsystem. Sub-images are collected over a period of time that correspondsto the time it takes a selected portion of anatomy to traverse ½ of theactive imaging volume. Thus at the end of the acquisition of asub-image, the center of the sub-image is ¾^(th) of the way into theactive volume of the imaging system. Upon the completion of thesub-image the next sub-image is initiated with an offset equal to 2 theFOV. Once all the sub-images have been collected the central portion ofeach sub-image is extracted and combined with other central portions toform the composite image. As with the prior embodiment, extraction ofthe central portions of the image reduces the severity of “stitching”artifacts.

[0040] In another further embodiment, a single receiver is employedwherein the receiver is adapted to receive the plurality of MR signalsfrom rf coil 152 and is further adapted to be adjusted in at least oneof phase and frequency in response to translation the positioning devicethrough the magnet assembly. In this embodiment, the receiver is alsofurther adapted to collect image data for a field-of-view correspondingto a useable volume of the magnet assembly. Thereafter, processing ofthe MR signals is performed to compute a plurality of respectivesub-images for the field-of-view (FOV) at each of a plurality ofincremented positions as the subject is translated. A central portion ofeach of the plurality of respective sub-images is combined to form acomposite image of the volume of interest.

[0041] Referring to FIG. 4, an embodiment for the present inventionemploys a separate RF receive coil array 353 that complements RF coil152 (of FIG. 1). During moving table imaging, RF coil 152 is used fortransmission and coil array 353 is used for reception. Arranging coilarray elements 300 of coil array 353 (combination of elements 300comprises the receive coil array) such that they distribute indimensions orthogonal relative to the frequency encoding direction isdesirable for reducing phase encoding steps and speeding up the imaging.For moving table imaging methods that frequency-encode along the tabletranslation direction, a preferred embodiment employs coil array 353 ofcoil array elements 300 that are placed on a fixture 310 that wrapsaround the examined subject 200 (FIG. 1) but remains stationary withrespect to the scanner (see FIG. 4). Alternatively, the array of receivecoils 353 do not remain stationary and are moved concurrently with thetable motion. In either embodiment, during moving table imaging, thecoils in the array receive in parallel. A moving table imagingreconstruction algorithm that integrates the parallel imagingreconstruction concept is used to reconstruct full-FOV images. It is tobe appreciated that other rf coil array configuration, as are well-knownin the art, may be employed with methods of the present invention. Othersuch configurations may include arranging coil array elements 300 ofreceive coil array 353 such that they wrap around only the anteriorportion of subject 200, and/or additionally distribute along thefrequency encoding direction.

[0042] Unlike a body coil, the B1 field of any of the receive coil arrayelements may vary spatially and induce image artifacts. Addressing theartifacts will be described in greater detail below. The methods ofimaging (acquisition and reconstruction) as described earlier with amoving table are applicable when employing a receive coil array ratherthan a body coil.

[0043] In embodiments employing receive coil arrays, data acquisition isconducted concurrently with table translation. The receivers' centerfrequencies are adjusted, or alternatively swept, at a rate commensuratewith the table motion, which allows the scanner to follow a selected setof spins during each round of phase encodes. This enables each of thereceivers, or alternatively channels, to acquire a coherent k-space datamatrix and produce a regional image that maps out the set withnegligible effects of the concurrent translation. Reduction of k-spacesampling density is in effect in the present case, and leads to aliasingalong the phase encoding direction(s) in each of the regional images.Applying SENSE or other parallel imaging reconstruction on the regionalimages that are produced in parallel generates a regional image free ofaliasing. The process is repeated to produce a series of regional imagesof the volume of interest along the translation direction, which can becombined into a full-FOV image. In the presence of B1 fieldinhomogeneity however, the followed set of spins is “seen” by each coilwith a different sensitivity weighting at different table locations,especially for embodiments that employ a stationary coil array 153 toreceive signals during table translation. This leads to undesirableview-to-view changes that may cause significant ghosting artifacts. Twostrategies may be applied to alleviate/eliminate this problem. First,one may design the coil array elements 300 to assume shapes that extendthe full travel range of the selected spins, which is determined by thebandwidth of the signal filtration during each view acquisition, tabletranslation speed, and pulse sequence timing. This reduces B1 fieldvariation along the translation direction and hence lessens view-to-viewchanges. Second, one may, in the reconstruction, take into account boththe B1 field maps and the table locations and correct for theview-to-view changes algebraically.

[0044] Embodiments of the present invention provide for a MRI systemthat may be employed as a whole body screening tool for metastaticcancer and other diseases. In the embodiments for the methods ofextended volume imaging, various two-dimensional or three-dimensional MRpulse sequences (e.g. spin echo, fast spin echo, echo-planar, gradientecho, and FIESTA or FISP) may be employed. The imaging sequence may beone of multi-slice, multi-slab, and volume imaging sequences. Use ofmany MR pulse sequence is possible since only the phase or the frequencyof the receiver and/or transmitter is modified during a scan.Embodiments of the methods of the present invention allow the user toselect the desired direction of the phase and frequency encodingdirections in order to advantageously place phase-encoding artifacts indesired directions. The user is also able to select interleavedacquisition of multiple slices, if necessary.

[0045] In further embodiments, embodiments of the present invention areimplemented as a hardware subsystem in the scanner that is independentof the pulse sequence itself. That way any pre-existing imaging strategyand information content can be obtained over the entire subject. Ahardware system implementation would comprise incorporating a subsystemin the scanner that varies the receiver offset frequency or phase inresponse to changes in table position, or alternatively, changes thetable position in response to changes in receiver frequency/phase.

[0046] While the preferred embodiments of the present invention havebeen shown and described herein, it will be obvious that suchembodiments are provided by way of example only. Numerous variations,changes and substitutions will occur to those of skill in the artwithout departing from the invention herein. Accordingly, it is intendedthat the invention be limited only by the spirit and scope of theappended claims.

1. An imaging apparatus for producing Magnetic Resonance (MR) images of a subject, the apparatus comprising: a magnet assembly for producing a static magnetic field; a gradient coil assembly for generating a magnetic field gradient for use in producing MR images; at least one radiofrequency (rf) coil array disposed about the subject for transmitting a radiofrequency pulse and for detecting a plurality of magnetic resonance (MR) signals induced from the subject for a given imaging sequence; a positioning device for supporting the subject and for translating the subject during imaging; and, a plurality of receivers for receiving the plurality of MR signals, the receivers each being adapted to adjust their respective center frequencies at a rate commensurate with a rate of translation of the positioning device.
 2. The apparatus of claim 1 wherein the at least one rf coil array is mounted on a fixture that is disposed about the subject.
 3. The apparatus of claim 2 wherein the fixture and rf coil array mounted thereon are stationary relative to the static magnetic field.
 4. The apparatus of claim 2 wherein the fixture and rf coil array mounted thereon are moveable relative to the static magnetic field.
 5. The apparatus of claim 1 wherein the at least one rf coil array comprises a plurality of coil elements arranged in a orthogonal distribution relative to a frequency encoding direction.
 6. The apparatus of claim 1 wherein the at least one rf coil array detects the MR signals concurrently with the translation of the positioning device.
 7. The apparatus of claim 1 further comprising: an image processor for computing a plurality of respective sub-images corresponding to a field-of-view at a plurality of incremented locations of the subject and wherein the image processor is further adapted to combine the plurality of respective sub-images to form a composite image of the subject.
 8. The apparatus of claim 1 wherein the imaging sequence is one of multi-slice, multi-slab, and volume imaging sequences.
 9. A method for producing an image from an extended volume of interest within a subject using a Magnetic Resonance Imaging (MRI) system where the extended volume of interest is larger than an imaging portion of a magnet within the MRI system, the method comprising: translating the volume using a positioning device along an axis of the MRI system and imaging portions of the volume when they are within the imaging portion of the magnet; detecting a plurality of MR signals from at least one radiofrequency (RF) coil array for a given field-of-view within the MRI system as the positioning device is translating the volume; sending the plurality of MR signals to a plurality of receivers, the receivers each being adapted to adjust their respective center frequencies at a rate commensurate with a rate of translation of the positioning device, computing a plurality of respective sub-images corresponding to the plurality MR signals for each of the plurality of receivers and for the given field-of-view (FOV) at a plurality of incremented locations of the subject; and, combining the plurality of respective sub-images to form a composite image of the volume of interest.
 10. The method of claim 9 wherein the at least one rf coil array is mounted on a fixture that is disposed about the subject.
 11. The method of claim 10 wherein the fixture and rf coil array mounted thereon are stationary relative to the static magnetic field.
 12. The method of claim 10 wherein the fixture and rf coil array mounted thereon are moveable relative to the static magnetic field.
 13. The method of claim 9 wherein the at least one rf coil array comprises a plurality of coil elements arranged in a orthogonal distribution relative to a frequency encoding direction.
 14. The method of claim 9 wherein the detecting step is performed concurrently with the translating step.
 15. The method of claim 9 wherein the translating step is repeated until a selected length of the subject has been imaged inside the imaging portion of the magnet.
 16. The method of claim 9 wherein the combining step further comprises combining a central portion of each sub-image to form the composite image.
 17. The method of claim 9 wherein the extended volume of interest is a head-to-toe view of the subject.
 18. A method for imaging an extended volume of interest within a subject using a Magnetic Resonance Imaging (MRI) system comprising: translating the subject into an imaging portion of a magnet assembly of the MRI system; detecting a plurality of MR signals from a radiofrequency (RF) coil array; and, sending the plurality of MR signals to a plurality of receivers, the receivers each being adapted to adjust their respective center frequencies at a rate commensurate with a rate of translation of the positioning device; and, reconstructing at least one image of the volume of interest by computing a plurality of respective sub-images corresponding to the plurality MR signals for each of the plurality of receivers and for the given field-of-view (FOV) at a plurality of incremented locations of the subject as the subject is translated and combining the plurality of respective sub-images to form a composite image of the volume of interest.
 19. The method of claim 18 wherein the extended volume of interest is a head-to-toe view of the subject.
 20. The method of claim 18 wherein the at least one rf coil array comprises a plurality of coil elements arranged in orthogonal distribution to a frequency encoding direction.
 21. The method of claim 18 wherein the at least one rf coil array is mounted on a fixture that is disposed about the subject.
 22. The method of claim 21 wherein the fixture and rf coil array mounted thereon are stationary relative to the static magnetic field.
 23. The method of claim 21 wherein the fixture and rf coil array mounted thereon are moveable relative to the static magnetic field.
 24. The method of claim 18 wherein the detecting step is performed concurrently with the translating step.
 25. The method of claim 18 wherein the translating step is repeated until a selected length of the subject has been imaged.
 26. The method of claim 18 wherein the translating step is substantially continuous. 